Method and system for mitigating urea deposits within an scr catalyst system

ABSTRACT

A method and system for mitigating a urea deposit within an SCR system that includes determining a mass of an accumulated urea deposit present within the SCR catalyst and SCR piping, comparing the mass of the accumulated urea deposit with a deposit upper threshold limit, and initiating an SCR regeneration event when the mass of the accumulated urea deposit is greater than the deposit upper threshold limit. The method further includes determining an amount of NH 3  passing through the SCR catalyst downstream of the urea deposit, comparing the amount of NH 3  passing through the SCR catalyst with an NH 3  regeneration threshold limit, and terminating the SCR regeneration event when the level of NH 3  passing through the SCR catalyst is less than the SCR NH 3  regeneration threshold.

FIELD

This disclosure relates to controlling nitrogen oxides (NO_(x))emissions for internal combustion engines, and more particularly toapparatus, systems and methods for estimating and mitigating ureadeposits for a selective catalytic reduction (SCR) catalyst system.

BACKGROUND

Emissions regulations for internal combustion engines have become morestringent over recent years. Environmental concerns have motivated theimplementation of stricter emission requirements for internal combustionengines throughout much of the world. Governmental agencies, such as theEnvironmental Protection Agency (EPA) in the United States, carefullymonitor the emission quality of engines and set acceptable emissionstandards, to which all engines must comply. Consequently, the use ofexhaust after-treatment systems on engines to reduce emissions isincreasing.

Generally, emission requirements vary according to engine type. Emissiontests for compression-ignition (diesel) engines typically monitor therelease of carbon monoxide (CO), unburned hydrocarbons (UHC), dieselparticulate matter (PM) such as ash and soot, and nitrogen oxides(NO_(x)). Diesel oxidation catalysts (DOC) have been implemented inexhaust gas after-treatment systems to oxidize at least some particulatematter in the exhaust stream and to reduce the unburned hydrocarbons andCO in the exhaust to less environmentally harmful compounds. To removethe diesel particulate matter, a diesel particulate matter filter, or PMfilter, is typically installed downstream from the DOC or in conjunctionwith the DOC.

A common PM filter comprises a porous ceramic matrix with parallelpassageways through which exhaust gas passes. Particulate mattersubsequently accumulates on the surface of the filter, creating abuildup which must eventually be removed to prevent obstruction of theexhaust gas flow. The common forms of particulate matter are ash andsoot. Ash, typically a residue of burnt engine oil, is substantiallyincombustible and builds slowly within the filter. Soot, chieflycomposed of carbon, results from incomplete combustion of fuel andgenerally comprises a large percentage of particulate matter buildup.Various conditions, including, but not limited to, engine operatingconditions, mileage, driving style, terrain, etc., affect the rate atwhich particulate matter accumulates within a diesel particulate filter.Accumulation of particulate matter in the PM filter causes backpressureto build within the exhaust system. Excessive backpressure on the enginecan degrade engine performance (e.g., lower power and efficiency), andin some cases may result in engine stall.

Particulate matter, in general, oxidizes in the presence of nitricoxides (particularly NO₂) at modest temperatures, or in the presence ofoxygen at higher temperatures. If too much particulate matter hasaccumulated when oxidation begins, the oxidation rate may get highenough to cause an uncontrolled high-temperature excursion. Theresulting heat can destroy the filter and damage surrounding structures.Recovery can be an expensive process. To prevent potentially hazardoussituations, accumulated particulate matter is commonly oxidized andremoved in a controlled regeneration process before excessive levelshave accumulated.

For oxidation of the accumulated particulate matter, exhaust gastemperatures generally must exceed the temperatures typically reached atthe filter inlet. Consequently, additional methods to initiateregeneration of a diesel particulate filter may be used. In one method,a reactant, such as diesel fuel, is introduced into an exhaustafter-treatment system to increase the temperature of the exhaust gasesentering filter and initiate oxidation of particulate buildup. A PMfilter regeneration event occurs when substantial amounts of soot areconsumed on the PM filter.

A controlled PM filter regeneration can be initiated by the engine'scontrol system when a predetermined amount of particulate hasaccumulated on the filter, when a predetermined time of engine operationhas passed, or when the vehicle has driven a predetermined number ofmiles. Controlled or active PM filter regeneration typically consists ofdriving the filter temperature up to O₂ oxidation temperature levels fora predetermined time period such that oxidation of soot accumulated onthe PM filter takes place.

With regard to reducing NO_(x) emissions, NO_(x) reduction catalysts,including selective catalytic reduction (SCR) systems, are utilized toconvert NO_(x) (NO and NO₂ in some fraction) to N₂ and other compounds.SCR systems utilize a reductant, typically ammonia, to reduce theNO_(x). Currently available SCR systems can produce high NO_(x)conversion rates allowing the combustion technologies to focus on powerand efficiency. However, currently available SCR systems also sufferfrom a few drawbacks.

SCR systems utilize a spray dosing system to introduce ammonia reductantinto the exhaust stream upstream of the SCR catalyst. When just theproper amount of ammonia is available at the SCR catalyst under theproper conditions, the ammonia is utilized to reduce NO_(x). However, ifthe reduction reaction rate is too slow, or if excess ammonia isintroduced into the exhaust stream upstream of the SCR catalyst, thesurplus ammonia can slip through the SCR without being utilized in thecatalytic reaction. Because ammonia is an extreme irritant and anundesirable emission, ammonia slips of even a few tens of ppm areproblematic. As a result, an additional ammonia oxidation (AMOX)catalyst may be installed downstream of the SCR catalyst to capture andoxidize any residual ammonia components in the exhaust stream into morebenign compounds prior to being released into the atmosphere.

Due to the undesirability of handling pure ammonia, many systems utilizean alternate solution, such as aqueous urea, that vaporizes anddecomposes to ammonia in the exhaust stream. However, the decompositionof urea into ammonia can be highly dependent upon the temperature of theexhaust gas. If the temperature of the exhaust gas is too low, forexample, some of the vaporized urea can condense onto the interiorsurfaces of the SCR system piping and the SCR catalyst bed andcrystallize into urea deposits, which can build over time.

Like the build-up of particulate matter in the PM filter, the ureadeposits in the SCR system may also have negative impacts on theoperation of the internal combustion engine. For instance, the depositsmay restrict the flow passages of the exhaust stream, causing higherback-pressures and reducing engine and after-treatment systemperformance and efficiency. The deposits may also disrupt the flow andmixing of the urea reductant into the exhaust stream and thereby reducethe decomposition into ammonia (NH₃) with a subsequent drop in NO_(x)reduction efficiency. Moreover, the re-direction of a portion of theinjected urea into the urea deposits can also reduce the amount of ureareductant which was intended to reach the SCR catalyst, making controlof the SCR system more difficult and further reducing NO_(x) reductionefficiency.

Similar to the periodic controlled oxidation or regeneration of the PMfilter to remove an excess accumulation of particulate matter, anincrease in the temperature of the exhaust gases entering the SCR systemcan release and decompose the urea deposits into ammonia for utilizationor storage in the SCR catalyst. However, if the urea deposits have beenallowed to grow too large, the additional surge of ammonia can be inexcess of the amount which can be effectively utilized in the ongoingcatalytic reaction or captured and stored in the SCR catalyst, and thuswill escape the SCR system as ammonia slip. In addition, the repeatedcycling of the SCR system inlet temperature to temperature levelssufficient to release and decompose the urea deposits into ammonia canboth reduce the efficiency of the ongoing SCR reaction as well as causepremature wear and degradation of the SCR catalyst components, andshould be limited as much as possible.

Also, many known SCR systems do not utilize an ammonia oxidation (AMOX)catalyst downstream of the SCR catalyst to convert at least some ammoniaslipping from the SCR catalyst to N₂ and other less harmful compounds.For those conventional SCR systems that do not include an AMOX catalyst,the operating conditions and conversion capability of the AMOX catalystare not factored into the reductant dosing rate, ammonia storagecontrol, ammonia slippage control, and NO_(x) conversion efficiencyfeedback of such systems. However, these conventional AMOX-less SCRsystems are still required to comply with the stringent emissionrequirements for internal combustion engines, and must meet thesestandards through different means.

SUMMARY

The subject matter of the present application has been developed inresponse to the present state of the art, and in particular, in responseto the problems and needs in the art that have not yet been fully solvedby currently available exhaust after-treatment systems. Accordingly, thesubject matter of the present application has been developed to providemethods and systems for estimating and mitigating urea deposits withinan SCR system that overcomes at least some shortcomings of the prior artafter-treatment systems. More specifically, disclosed herein is animproved method and system for determining the build-up of the ureadeposits within the SCR system with greater accuracy. In addition, amore capable SCR regeneration control system is described herein thatbetter controls the SCR regeneration process so as to minimize the highinlet temperature regeneration cycles while maintaining the ureadeposits to a minimum amount, which helps to avoid any excess ammoniaslip during the SCR regeneration cycle.

According to one representative embodiment, a method for mitigating aurea deposit within a selective catalytic reduction (SCR) system that isin exhaust receiving communication with an exhaust gas stream producedby an engine system can include various steps or operations. Forexample, the method can include determining a mass of an accumulatedurea deposit present within the SCR system, comparing the mass of theaccumulated urea deposit with a deposit upper threshold limit, andinitiating an SCR regeneration event when the mass of the accumulatedurea deposit is greater than the deposit upper threshold limit. Themethod can further include determining an amount of ammonia (NH₃)passing through the SCR catalyst downstream of the urea deposit,comparing the amount of NH₃ passing through the SCR catalyst with an NH₃regeneration threshold limit, and terminating the SCR regeneration eventwhen the level of NH₃ passing through the SCR catalyst is less than theNH₃ regeneration threshold limit.

In certain implementations, determining the mass of the accumulated ureadeposit includes determining a rate of urea deposit accumulation withinthe SCR system, determining a first time period, the first time periodbeing the time elapsed from a previous SCR regeneration event, andintegrating the rate of urea deposit accumulation within the SCR systemover the first time period to calculate the mass of the accumulated ureadeposit. The method may also include measuring the flow rate of NH₃passing through the SCR system with an NH₃ sensor positioned downstreamof the urea deposit. In some implementations, the NH₃ sensor is an SCRcatalyst NH₃ mid-bed sensor.

According to another representative embodiment, a method for mitigatingthe urea deposit within the selective catalytic reduction (SCR) systemcan include determining a mass of a urea deposit present within the SCRcatalyst, comparing the accumulated mass of the urea deposit with adeposit upper threshold limit, and initiating an SCR regeneration eventwhen the accumulated mass of the urea deposit is greater than thedeposit upper threshold limit. The method can also include determiningan amount of NH₃ passing through the SCR catalyst downstream of the ureadeposit, comparing the amount of NH₃ passing through the SCR catalystwith an NH₃ regeneration threshold limit, and maintaining the SCRregeneration event when the level of NH₃ passing through the SCRcatalyst is greater than the NH₃ regeneration threshold limit. Themethod can further include comparing the mass of the urea deposit with adeposit lower threshold limit, and terminating the SCR regenerationevent when the mass of the decomposing urea deposit is less than thedeposit lower threshold limit.

In some implementations, determining the mass of the accumulated ureadeposit includes determining a rate of urea deposit accumulation withinthe SCR system, determining a first time period, the first time periodbeing the time elapsed from a previous SCR regeneration event, andintegrating the rate of NH₃ passing through the SCR catalyst over thesecond time period to calculate the amount of NH₃ passing through theSCR catalyst. In certain implementations, determining the mass of theaccumulated urea deposit includes determining a rate of urea depositdecomposition within the SCR system and integrating the rate of ureadeposit decomposition within the SCR system over the second time periodto calculate the second estimate of the mass of the accumulated ureadeposit.

According to yet another representative embodiment, a system formitigating a urea deposit within a selective catalytic reduction (SCR)system that is in exhaust receiving communication with an exhaust gasstream produced by an engine system can include various sensor andcontrol system modules. For example, the system can include a reductantdelivery system configured to inject reductant into the exhaust gasstream upstream of the SCR catalyst. The system can further include anSCR NH₃ sensor that is also in exhaust gas receiving communication withthe exhaust gas stream. The SCR NH₃ sensor can be located downstream ofthe accumulated urea deposit.

The system can also include a controller having an SCR catalystregeneration module that is in electrical communication with each of thereductant delivery system and the SCR NH₃ sensor. In turn, the SCRcatalyst regeneration module can include a urea deposit accumulationmodule that is operable to determine a mass of an accumulated ureadeposit present within the SCR catalyst from inputs received from thereductant delivery system (which can be a reductant dosing flow signal).The SCR catalyst regeneration module can also include an NH₃ slipfeedback module that is operable to determine an amount of NH₃ passingthrough the SCR catalyst downstream of the urea deposit from sensorinputs received from the SCR NH₃ sensor.

In addition, the SCR catalyst regeneration module may be operable toboth initiate an SCR regeneration event when the mass of the accumulatedurea deposit is greater than a deposit upper threshold limit, and toterminate the SCR regeneration event when the level of NH₃ passingthrough the SCR catalyst after initiation of the SCR regeneration eventis less than an SCR NH₃ regeneration threshold.

According to another embodiment, a computer program product thatincludes a computer readable medium with computer usable program codeexecutable to perform operations for mitigating a urea deposit within anSCR catalyst included within and SCR system. The SCR system is coupledto an internal combustion engine and operable to inject a reductant,such as urea, into an exhaust gas stream. While most of the injectedurea reduces to ammonia in the exhaust gas stream and facilitates areduction of NO_(x) emissions in the exhaust gas stream within the SCRcatalyst, a portion of the reductant can condense and crystallize ontothe surfaces of the SCR piping and the SCR catalyst, thereby forming aproblematic urea deposit.

The operations of the computer program product can include determining amass of an accumulated urea deposit present within the SCR catalyst,comparing the mass of the accumulated urea deposit with a deposit upperthreshold limit, and initiating an SCR regeneration event when the massof the accumulated urea deposit is greater than the deposit upperthreshold limit. The operations of the computer program product canfurther include determining an amount of NH₃ passing through the SCRcatalyst downstream of the urea deposit, comparing the amount of NH₃passing through the SCR catalyst with an NH₃ regeneration thresholdlimit, and terminating the SCR regeneration event when the level of NH₃passing through the SCR catalyst is less than the SCR NH₃ regenerationthreshold.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the subject matter of the present disclosureshould be or are in any single embodiment. Rather, language referring tothe features and advantages is understood to mean that a specificfeature, advantage, or characteristic described in connection with anembodiment is included in at least one embodiment of the presentdisclosure. Thus, discussion of the features and advantages, and similarlanguage, throughout this specification may, but do not necessarily,refer to the same embodiment.

The described features, structures, advantages, and/or characteristicsof the subject matter of the present disclosure may be combined in anysuitable manner in one or more embodiments and/or implementations. Inthe following description, numerous specific details are provided toimpart a thorough understanding of embodiments of the subject matter ofthe present disclosure. One skilled in the relevant art will recognizethat the subject matter of the present disclosure may be practicedwithout one or more of the specific features, details, components,materials, and/or methods of a particular embodiment or implementation.In other instances, additional features and advantages may be recognizedin certain embodiments and/or implementations that may not be present inall embodiments or implementations. Further, in some instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the subject matter ofthe present disclosure. The features and advantages of the subjectmatter of the present disclosure will become more fully apparent fromthe following description and appended claims, or may be learned by thepractice of the subject matter as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the subject matter may be more readilyunderstood, a more particular description of the subject matter brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the subject matter and arenot therefore to be considered to be limiting of its scope, the subjectmatter will be described and explained with additional specificity anddetail through the use of the drawings, in which:

FIG. 1 is a schematic diagram of an engine system having an internalcombustion engine and an exhaust after-treatment system, in accordancewith one representative embodiment;

FIG. 2 is a schematic diagram of the exhaust after-treatment system ofFIG. 1, in accordance with one representative embodiment;

FIG. 3 is a schematic block diagram of a controller of the engine systemof FIG. 1, in accordance with one representative embodiment;

FIG. 4 is a schematic block diagram of a controller of the engine systemof FIG. 1, in accordance with another representative embodiment;

FIG. 5 is a schematic block diagram of an SCR catalyst regenerationmodule of the controller of FIG. 3, in accordance with onerepresentative embodiment; and

FIG. 6 is a schematic flow chart diagram of a control system operable todetermine and remove an accumulation of urea deposits, in accordancewith one representative embodiment.

DETAILED DESCRIPTION Internal Combustion Engine System

FIG. 1 depicts one embodiment of an engine system 10. The maincomponents of the engine system 10 include an internal combustion engine20 and an exhaust gas after-treatment system 100 in exhaustgas-receiving communication with the engine 20. The internal combustionengine 20 can be a compression-ignited internal combustion engine, suchas a diesel fueled engine, or a spark-ignited internal combustionengine, such as a gasoline fueled engine operated lean. On the intakeside, the engine system 10 can include an air inlet 12, inlet piping 14,a turbocharger compressor 16, and an intake manifold 18. The intakemanifold 18 includes an outlet operatively coupled to compressionchambers 22 of the internal combustion engine 20 for introducing airinto the compression chambers 22.

Within the internal combustion engine 20, the air from the atmosphere iscombined with fuel to power the engine. The fuel comes from the fueltank 50 through a fuel delivery system including, in one embodiment, afuel pump and common rail 52 to the fuel injectors 54, which inject fuelinto the combustion chambers 22 of the engine 20. Fuel injection timingcan be controlled by the controller 40 via a fuel injector controlsignal 84.

Combustion of the fuel and air in the compression chambers 22 producesexhaust gas that is operatively vented to an exhaust manifold 30. Fromthe exhaust manifold 30, a portion of the exhaust gas may be used topower the turbocharger turbine 32. The turbocharger turbine 32 drivesthe turbocharger compressor 16, which may compress at least some of theair entering the air inlet 12 before directing it to the intake manifold18 and into the compression chambers of the engine 20.

For the purposes of altering the combustion properties of the engine 20,a portion of the exhaust gas may bypass the turbocharger turbine 32 andbe re-circulated to the engine 20 via an exhaust gas recirculation (EGR)line 36 and back to the inlet piping 14. In one embodiment, an EGR valve38 is actuated to divert an amount of exhaust gas corresponding to aproportion set by a controller 40 via an EGR control signal.

The portion of the exhaust gas which is not re-circulated to the engine110 via the EGR line 36 is destined for expulsion from the engine system10 into the atmosphere. Thus, the exhaust gas stream flows from theexhaust manifold 30 or turbocharger turbine 32, through the exhaustpiping 34, and through the exhaust gas after-treatment system 100 priorto being vented into the atmosphere through tailpipe 35. The exhaust gasafter-treatment system 100 is configured to remove various chemicalcompounds and particulate emissions present in the exhaust gas receivedfrom the exhaust manifold 30. Specifically, the exhaust gas treated inthe exhaust gas after-treatment system 100 contains significantly fewerpollutants, such as unburned hydrocarbons, CO, diesel particulatematter, and NO_(x), than untreated exhaust gas.

Various sensors, such as temperature sensors 64, pressure sensors 66,fuel sensor 72, exhaust gas flow sensors 74, 76 and the like, may bestrategically disposed throughout the engine system 10 and may be incommunication with the controller 40 to monitor operating conditions ofthe engine system 10. In one embodiment, the fuel sensor 72 senses theamount of fuel consumed by the engine, and the exhaust gas flow sensor74 senses the rate at which the exhaust gas is flowing towards exhaustgas after-treatment system 100.

Exhaust Gas After-Treatment System

Referring now to FIG. 2, the exhaust gas after-treatment system 100includes the controller 40, an oxidation catalyst 140, a particulatematter (PM) filter 142, a selective catalytic reduction “SCR” system 150having a reductant delivery system 151 and an SCR catalyst 152, and anoptional ammonia oxidation (AMOX) catalyst 154. In an exhaust flowdirection, as indicated by directional arrow 144, exhaust may flow fromthe exhaust piping 36, through the oxidation catalyst 140, through thePM filter 142, through the SCR catalyst 152, through the AMOX catalyst160, and then be expelled into the atmosphere through tailpipe 35. Thus,in the illustrated embodiment the PM filter 142 is positioned downstreamof the oxidation catalyst 140, the SCR catalyst 152 is positioneddownstream of the PM filter 142, and the AMOX catalyst 154 is positioneddownstream of the SCR catalyst 152. However, other arrangements of thecomponents of the exhaust gas after-treatment system 100 are alsopossible.

The oxidation catalyst 140 can be any of various flow-through designsknown in the art, including diesel oxidation catalysts (DOC). Generally,the oxidation catalyst 140 is configured to oxidize at least someparticulate matter, e.g., the soluble organic fraction of soot, in theexhaust and reduce unburned hydrocarbons and CO in the exhaust to lessenvironmentally harmful compounds. For example, the oxidation catalyst140 may sufficiently reduce the hydrocarbon and CO concentrations in theexhaust to meet the requisite emissions standards for those componentsof the exhaust gas.

In addition to treating the hydrocarbon and CO concentrations in theexhaust gas, the oxidation catalyst 140 can also be used in thecontrolled regeneration of the PM filter 142 and the SCR catalyst 152.This can be accomplished through the injection, or dosing, of unburnedhydrocarbons “UHC” into the exhaust gas upstream of the oxidationcatalyst 140. Upon contact with the oxidation catalyst 140, the UHCundergoes an exothermic oxidation reaction which leads to an increase inthe temperature of the exhaust gas exiting the oxidation catalyst 140and subsequently entering the PM filter 142 and/or SCR catalyst 152. Theamount of UHC added to the exhaust gas is selected to achieve thedesired temperature increase or target controlled regenerationtemperature.

The engine system 10 of FIG. 1 can use an internal fuel dosing strategy,an external fuel dosing strategy, or a combination of internal andexternal fuel dosing strategies to introduce the desired amount of UHCinto the primary exhaust gas stream prior to the oxidation catalyst 140.In an internal fuel dosing strategy, for instance, the UHC can bederived from extra fuel that is injected into the compression cylinders22 of the engine 20 after the combustion events having taken placewithin the cylinder. The UHC are then carried out of the engine 20 alongwith the products of combustion to the exhaust manifold 30, and fromthere to the exhaust gas after-treatment system 100. The internal fuelinjection approach can be the same as or similar to the approachdescribed in U.S. Pat. No. 8,061,127, which is incorporated herein byreference.

In an external fuel dosing strategy, the UHC can be derived from extrafuel that is injected via a separate fuel injector 56 directly into theexhaust piping 34 upstream of the exhaust gas after-treatment system100. The fuel injector 56 may be in fluid communication with the fuelsource 50 and selectively activated as needed based on an external fueldosing control signal 86 received from the controller 40. The externalfuel injection approach can be the same as or similar to the approachdescribed in U.S. Pat. No. 7,263,825, which is incorporated herein byreference. Although fuel dosing strategies have been described as typesof thermal management levers for increasing the exhaust temperature,other levers can be used, such as exhaust heaters and internal enginecombustion strategies.

The PM filter 142 can be any of various flow-through designs known inthe art, including diesel particulate filters (DPF), and configured toreduce particulate matter concentrations, e.g., soot and ash, in theexhaust gas to meet requisite emission standards. In addition, theexhaust gas after-treatment system 100 can further include a variety ofsensors surrounding the PM filter 142 and which are electrically coupledto the controller 40. The sensors can include exhaust gas temperaturesensors 164B, 164C and exhaust gas pressure sensors 166A, 166B.

Based on the input from these sensors, the controller 40 can monitorvarious properties of the exhaust gas entering and leaving the PM filter142 so as to ascertain the current status of the PM filter 142. Theseexhaust gas properties may include the change in pressure across the PMfilter 142 and the temperature of the exhaust gas stream entering andleaving the PM filter 142. Using these values, the controller 40 canmonitor the temperature, timing, duration and effectiveness of filterregeneration events. In some implementations, the PM filter 142 andassociated sensors and control system 40 is similar to, or the same as,the respective particulate filters, sensors, and control systemsdescribed in U.S. Pat. No. 7,231,291, U.S. Pat. No. 7,562,524, U.S. Pat.No. 7,478,527, and U.S. Pat. No. 7,587,892, each of which isincorporated herein by reference.

The SCR system 150 includes a reductant delivery system 151 comprising areductant source 170, pump 180 and delivery mechanism 190. The reductantsource 170 can be a container or tank capable of retaining a reductant,such as, for example, ammonia (NH₃), urea, diesel fuel, or diesel oil.The reductant source 170 is in reductant supplying communication withthe pump 180, which is configured to pump reductant from the reductantsource to the delivery mechanism 190. The delivery mechanism 190 caninclude a reductant injector schematically shown at 192 positionedupstream of the SCR catalyst 152. The injector is selectivelycontrollable to inject reductant directly into the exhaust gas streamprior to entering the SCR catalyst 152.

In some embodiments, the reductant can either be ammonia or urea, whichdecomposes to produce ammonia. As will be described in more detailbelow, in these embodiments, the ammonia reacts with NO_(x) in thepresence of the SCR catalyst 152 to reduce the NO_(x) to less harmfulemissions, such as N₂ and H₂O. The SCR catalyst 152 can be any ofvarious catalysts known in the art. For example, in someimplementations, the SCR catalyst 152 is a vanadium-based catalyst, andin other implementations, the SCR catalyst is a zeolite-based catalyst,such as a Cu-Zeolite or a Fe-Zeolite catalyst. In one representativeembodiment, the reductant is aqueous urea and the SCR catalyst 152 is azeolite-based catalyst.

The AMOX catalyst 154 can be any of various flow-through catalystsconfigured to react with ammonia to produce mainly nitrogen. Generally,the AMOX catalyst 154 is utilized to remove ammonia that has slippedthrough or exited the SCR catalyst 152 without reacting with NO_(x) inthe exhaust. In certain instances, the after-treatment system 100 can beoperable with or without an AMOX catalyst. Further, although the AMOXcatalyst 154 is shown as a separate unit from the SCR catalyst 152, insome implementations, the AMOX catalyst can be integrated with the SCRcatalyst, e.g., the AMOX catalyst and the SCR catalyst can be locatedwithin the same housing.

The embodiment of the exhaust after-treatment system 100 illustrated inFIG. 2 incorporates multiple NO_(x) sensors which measure the reductionof NO_(x) throughout the exhaust treatment process. These include NO_(x)sensor 162A upstream of the oxidation catalyst 140, NO_(x) sensor 162Bembedded within the SCR catalyst 152, NO_(x) sensor 162C intermediatethe SCR catalyst 152 and the AMOX catalyst 154, and NO_(x) sensor 162Ddownstream of the AMOX catalyst 154.

The exhaust after-treatment system 100 can also utilize various sensorssurrounding or even installed within the SCR catalyst and the AMOXcatalyst, including temperature sensors 164D-164F, oxygen sensor 178,NH₃ sensors 168A-168C, and the like. As shown, NH₃ sensor 166A islocated upstream of the SCR catalyst 152, NH₃ sensor 166B is embeddedwithin the SCR catalyst 152, and NH₃ sensor 166C downstream of the AMOXcatalyst 154. In some cases an NO_(x) sensor and an NH₃ sensor may becombined into a dual ammonia-NO_(x) sensors (not shown). The varioussensors may be in electrical communication with the controller 40 toallow the controller monitor the operating conditions of the exhaustafter-treatment system 100 of the engine system 10.

Although the exhaust after-treatment system 100 shown includes one of anoxidation catalyst 140, PM filter 142, SCR catalyst 152, and AMOXcatalyst 154 positioned in specific locations relative to each otheralong the exhaust flow path, in other embodiments, the exhaustafter-treatment system may include more than one of any of the variouscatalysts positioned in any of various positions relative to each otheralong the exhaust flow path as desired. Further, although the oxidationcatalyst 140 and AMOX catalyst 154 are non-selective catalysts, in someembodiments, the oxidation and AMOX catalysts can be selectivecatalysts.

The controller 40 controls the operation of the engine system 10 andassociated sub-systems, such as the internal combustion engine 20 andthe exhaust gas after-treatment system 100. The controller 40 isdepicted in FIGS. 1 and 2 as a single physical unit, but can include twoor more physically separated units or components in some embodiments ifdesired. Generally, the controller 40 receives multiple inputs,processes the inputs, and transmits multiple outputs. The multipleinputs may include sensed measurements from the sensors and various userinputs. The inputs are processed by the controller 40 using variousalgorithms, stored data, and other inputs to update the stored dataand/or generate output values. The generated output values and/orcommands are transmitted to other components of the controller and/or toone or more elements of the engine system 100 to control the system toachieve desired results, and more specifically, achieve desired exhaustgas emissions.

For example, the operating conditions of the internal combustion engine20 and the exhaust gas after-treatment system 100 can be ascertainedfrom any of the sensors or from the controller's 40 commands to theengine regarding the fraction of exhaust gas recirculation, injectiontiming, and the like. In one embodiment, information is gatheredregarding, for example, fuel rate, engine speed, engine load, the timingat which fuel injection timing is advanced or retarded (SOI, or start ofinjection), the fraction of exhaust gas recirculation, drivingconditions, exhaust flow rate, the amount of O₂, NO₂, and NH₃ and in theexhaust gas, exhaust gas temperatures and pressures at various locationswithin the exhaust gas after-treatment system 100, and the elapsed timesince one or more earlier events, such as a PM filter regeneration eventor an SCR catalyst regeneration event.

The controller 40 includes various modules for controlling the operationof the engine system 10. For example, the controller 40 includes one ormore modules for controlling the operation of the internal combustionengine 20. The controller 40 also includes one or more modules forcontrolling the regeneration of the PM filter 142 as described above.The controller 40 further includes one or more modules for controllingthe operation and regeneration of the SCR system 150. Additionally, inthe event the oxidation catalyst 140 and AMOX catalyst 154 areselectively controllable, the controller 40 can include one or moremodules for controlling the operation of the respective oxidation andAMOX catalysts.

Control System

FIG. 3 depicts a control system 200, according to one representativeembodiment. The control system 200 includes the controller 40, inputs 60from a variety of engine system and exhaust system sensors, and commandoutputs 80 to a variety engine system and exhaust system actuators. Thecontrol system inputs 60 may include sensor signals from the temperaturesensors 64, pressure sensors 66, fuel sensor 72 and exhaust gas flowsensors 74 of FIG. 1. The control system inputs 60 may also includesensor signals from the NO_(x) sensors 162A-162D, temperature sensors164A-164F, pressure sensors 166A-166B, NH₃ sensors 168A-168C, fuelsensor 172, and oxygen sensor 178 of FIG. 2. Additional sensor inputswhich are not specifically identified and described with reference toFIGS. 1-2 are also possible. The command outputs 80 may include, but arenot limited to, one or more engine system actuator control signals 84,one or more engine fuel injector control signals 86, a regeneration fuelinjector control signal 86, and an SCR dosing pump control signal 88.Furthermore, each of the control signals 82, 84, 86, 88 can include afeedback signal from the actuated component to the controller that isreflective of the current state or status of the actuated component.

The controller 40 includes various controller modules for controllingthe operation of the engine system 10. For example, in the illustratedembodiment the controller 40 includes an input module 210, a conditionsmodule 220, an output module 230, an SCR reductant dosing module 240, aPM filter regeneration module 250, an SCR catalyst regeneration module260, and a thermal management module 270. While not specificallyillustrated and described with reference to FIG. 3, the additionalcontroller modules for conducting other control system functions arealso possible and can be considered to fall within the scope of thepresent disclosure.

As is known in the art, the controller 40 and its various modularcomponents may comprise processor, memory, and interface modules thatmay be fabricated of semiconductor gates on one or more semiconductorsubstrates. Each semiconductor substrate may be packaged in one or moresemiconductor devices mounted on circuit cards. Connections between themodules may be through semiconductor metal layers,substrate-to-substrate wiring, or circuit card traces or wiresconnecting the semiconductor devices.

As mentioned above, the sensors of the engine system 10 are configuredto determine, whether directly or indirectly, a plurality of conditionswithin the engine system 10, including but not limited to temperature,pressure, NO_(x) concentration, exhaust gas flow rate and the like.Similarly, the actuators of the engine system 10 are any of variouscomponents that are selectively controlled or actuated to affect theoperating conditions of the system, including but not limited to anexhaust throttle, an intake throttle, the EGR valve and the like, withthe actuation of the engine fuel injectors, the regeneration fuelinjector, and the SCR dosing pump being shown separately in FIG. 3.

The input module 210 is configured to receive sensor inputs 60reflective of the conditions sensed by the various sensors, as well asactuator state inputs 80 from the engine system actuators, the engineand reductant fuel injectors, and the SCR dosing pump. The input module210 can then provide corresponding inputs to the SCR reductant dosingmodule 230, the PM filter regeneration module 250, the SCR catalystregeneration module 260, and the thermal management module 270. Theconditions module 220 is configured to gather additional informationregarding current conditions of the engine system 10, based on theconditions sensed by the sensors and/or other inputs including thecommand outputs 80 issued to the various system components by thecontroller 40. Via the command outputs 80, the output module 240 isconfigured to direct the engine system actuators, the engine fuelinjectors, the SCR dosing pump, and the regeneration fuel injectoraccording to the dosing and regeneration instructions generated by theSCR reductant dosing module 240, the PM filter regeneration module 250,the SCR catalyst regeneration module 260, and thermal management module270.

FIG. 4 depicts another representative embodiment of the control system300, which control system 300 also includes the controller 40, theinputs 60 from the various sensors, and the command outputs 80 to thevarious engine system and exhaust system actuators. In this embodiment,the controller 40 is depicted as comprising a processor module 310,memory module 320, and interface module 330. The processor module 310,memory module 320, and interface module 330 may be fabricated ofsemiconductor gates on one or more semiconductor substrates. Eachsemiconductor substrate may be packaged in one or more semiconductordevices mounted on circuit cards. Connections between the processormodule 310, the memory module 320, and the interface module 330 may bethrough semiconductor metal layers, substrate to substrate wiring, orcircuit card traces or wires connecting the semiconductor devices.

The memory module 320 stores software instructions and data comprisingone or more software processes. The processor module 310 executes thesoftware processes as is known to those skilled in the art. In theillustrated embodiment of FIG. 4, the processor module 310 can executeone or more software processes carried out by the conditions module 220,the SCR reductant dosing module 240, the PM filter regeneration module250, the SCR catalyst regeneration module 260, and the thermalmanagement module 270 of FIG. 3.

The processor module 310 may communicate with and control externaldevices and sensors, such as the sensors, the engine system actuators,the engine and reductant fuel injectors, and the SCR reductant dosingpump through the interface module 330. For example, the sensors mayinclude pressure sensors 156A and 156B (see, e.g., FIG. 2) thatcommunicate analog signals representing the pressure values upstream anddownstream of the PM filter 242 to the interface module 330. Theinterface module 330 may periodically convert the analog signals to adigital value and communicate the digital values to the processor module310.

The interface module 330 may also receive one or more digital signalsthrough a dedicated digital interface, a serial digital buscommunicating a plurality of digital values, or the like. For instance,the sensors may include one or more temperature sensors 64 (see, e.g.,FIG. 1) that communicate a digital temperature value to the interfacemodule 330. The interface module 330 may periodically communicate thedigital temperature value to the processor module 310. In oneembodiment, the interface module 330 executes one or more communicationprocesses carried out by the input module 210 and output module 230 ofFIG. 3.

The processor module 310 may store digital values such as the pressurevalue and the temperature value in the memory module 320. In addition,the processor module 310 may employ the digital values in one or morecalculations including calculations carried out by the various modulesof the controller 40. Although the above description was limited topressure and temperature sensors, the interface module 330 can interfacewith any of the various sensors of the engine system 10 and the exhaustafter-treatment system 100 to receive detected values for any of varioussensed characteristics of the system.

SCR Reductant Dosing Module

Referring back to FIGS. 2 and 3, the SCR reductant dosing module 240 isoperable to determine the amount of reductant which is required to bepumped into the exhaust after-treatment system 100 so as to reduce orsubstantially eliminate the NO_(x) components in the exhaust gas stream.The SCR reductant dosing module 240 can then direct the output module230 to output a command signal 88 to the reductant delivery system 151.The command signal 88 activates the reductant pump 180 to draw reductantfrom the reductant source 170 for injection via the delivery mechanism190 into the exhaust after-treatment system 100 upstream of the SCRcatalyst.

In some implementations, the SCR reductant dosing module 240 can be thesame as or similar to the combination of control modules described inU.S. Pat. No. 8,109,079, U.S. patent application Ser. No. 12/433,586,and U.S. Pat. No. 8,141,340, which are incorporated herein by reference.

PM Filter Regeneration Module

The PM filter regeneration module 250 is operable to determine thedegree of build-up of soot and ash in the PM filter 142 and to initiatea PM filter regeneration event, as described above. The PM filterregeneration module 250 can communicate a regeneration command to thethermal management module 270 that may include the desired PM filterinlet exhaust gas temperature, as measured by temperature sensor 164B.

In some implementations, the PM filter regeneration module 250 can bethe same as or similar to the combination of control modules describedin U.S. Pat. No. 7,263,825 and U.S. patent application Ser. No.12/111,831, as described above.

Thermal Management Module

Referring back to FIGS. 1-3, the thermal management module 270 isoperable to implement one or more unburned hydrocarbon (UHC) dosingstrategies based on a regeneration command received from the PM filterregeneration module or the SCR catalyst regeneration module. Theregeneration command may include, at least in part, the desired PMfilter inlet exhaust gas temperature received from the PM filterregeneration module 250, or a desired SCR catalyst inlet exhaust gastemperature received from the SCR catalyst regeneration module 260.

Based on the desired temperature included in the regeneration commandand the operating conditions of the engine 20 received from theconditions module 220, the thermal management module 270 can then directthe output module 230 to output a command signal 84 to the engine fuelinjectors 54 to initiate an internal fuel dosing event, or to output acommand signal 86 to the regeneration fuel injector 56 to initiate anexternal fuel dosing event. In the case of the external fuel dosingapproach, the command signal 86 activates the regeneration fuel injector56 to cause UHC from the fuel source 50 to be injected into the exhaustpiping 34 or the exhaust after-treatment system 100 upstream of theoxidation catalyst 140.

SCR Catalyst Regeneration Module

The SCR catalyst regeneration module 260 is operable to determine thedegree of build-up or amount of urea deposits in the SCR catalyst and toinitiate an SCR catalyst regeneration event to remove all or a portionof the deposits, as described above. In the present embodiment, the SCRcatalyst regeneration module 260 is configured to communicate an SCRcatalyst regeneration command to the thermal management module 270. TheSCR catalyst regeneration command may include the desired SCR catalystinlet exhaust gas temperature, as measured by temperature sensors 164Cor 164D.

As appreciated by one of skill in the art, the rate of urea depositaccumulation can vary substantially based on the temperature of theexhaust gas as it enters the SCR catalyst (e.g. which temperaturevariations are typically caused by the changing operating conditions ofthe internal combustion engine). Accordingly, the time interval betweenSCR regeneration events may also vary substantially. When thetemperature of the exhaust gas drops below some lower temperaturethreshold (e.g., when the engine system is in a slow idle mode, forexample), some of the vaporized urea can condense onto the interiorsurfaces of the SCR system piping and the SCR catalyst bed andcrystallize into urea deposits, which can build over time. Raising thetemperature of the exhaust gas to above some upper temperature threshold(e.g., such as during a SCR catalyst regeneration event) will cause thesurface of the urea deposit(s) to begin to sublimate or decompose backinto ammonia, which then continues through the SCR portion of theexhaust gas after-treatment system. It has been found that thetemperature of the exhaust gas at the SCR catalyst inlet must remainelevated for an extended determinable period of time for a urea depositof average mass to be completely decomposed into gaseous NH₃.

Referring to both FIGS. 2 and 5, the SCR catalyst regeneration module260 includes a urea deposit accumulation estimation module 262 and anNH₃ slip feedback module 264. The urea deposit accumulation estimationmodule 262 is configured to determine the rate of urea depositaccumulation within the SCR system. The urea deposit accumulationestimation module 262 may utilize any of various technologies ormethodologies for determining the rate of urea deposit accumulation.According to one implementation, the determination of the rate of ureadeposit accumulation is based, at least in part, on the urea dosing rateof the reductant delivery system 151 and/or other aftertreatmentoperating conditions. The urea dosing rate can be measured directly bythe reductant flow meter 172 or provided electronically by the SCRreductant dosing module 240.

To further refine its estimate for the rate of urea depositaccumulation, the urea deposit accumulation estimation module 262 mayalso utilize additional engine operating conditions 161, including, butnot limited to, the exhaust flow rate of the exhaust gas stream (fromexhaust flow sensor 74), SCR catalyst inlet NO_(x) flow rate (fromNO_(x) sensor 162B), and SCR catalyst outlet NO_(x) flow rate (fromNO_(x) sensor 162C). The inputs from each of the sensors can be passedto the urea deposit accumulation estimation module 262 via theconditions module 220.

The urea deposit accumulation estimation module 262 can apply the abovesensor inputs to an electronic model of the SCR system and SCR piping tocalculate the rate of urea deposit accumulation. Furthermore, theelectronic model may be calibrated for the particular characteristics ofthe SCR system and piping based on experimental data. Once the rate ofurea deposit accumulation has been calculated, the urea depositaccumulation estimation module 262 can then integrate the rate of ureadeposit accumulation over the elapsed time interval from a previous SCRregeneration event to determine the accumulated mass of the urea depositwhich is likely to be present within the SCR catalyst and SCR piping. Ofcourse, in other implementations, other methods and techniques, whichmay utilize actual sensors, virtual sensors, models, and/or the like,for determining the accumulated mass of the urea deposit within the SCRsystem can be used.

When the estimated accumulated mass of the urea deposit reaches apredetermined urea deposit threshold limit, also referred to as theupper threshold limit, the SCR catalyst regeneration module 260 isconfigured to initiate an SCR catalyst regeneration event bycommunicating an SCR catalyst regeneration command to the thermalmanagement module 270. The SCR catalyst regeneration command may includethe desired elevated value for the SCR catalyst inlet exhaust gastemperature, as measured by temperature sensors 164C or 164D. In someimplementations, the SCR catalyst regeneration command may also includea time period for which the SCR catalyst inlet exhaust gas is to bemaintained at the elevated temperature, which time period may bemeasured by a digital clock included within the controller 40.

As stated above, the thermal management module 270 is configured toreceive the SCR catalyst regeneration command and direct the outputmodule 230 to generate a command signal that initiates either aninternal fuel dosing event or an external fuel dosing regenerationevent. When the unburned hydrocarbons (UHC) come into contact with theoxidation catalyst 140, the resulting exothermic reaction raises thetemperature of the exhaust gas exiting the oxidation catalyst 140 andtraveling downstream towards the SCR catalyst 152. Based uponsimultaneous temperature measurements taken at temperature sensors 164Cor 164D, the amount of UHC added to the exhaust gas can be selectivelycontrolled by the thermal management module 270 to achieve and maintainthe target SCR catalyst inlet exhaust gas temperature needed forregeneration.

When the temperature of the SCR inlet gas reaches the desired elevatedvalue, the urea deposit will begin to sublimate and decompose back intoammonia, which then continues through the SCR catalyst towards the SCRmid-bed NH₃ sensor 168B. Generally, the amount of additional ammoniareleased by the decomposing urea deposit will be an order of magnitudegreater than the amount of ammonia sensed by the SCR mid-bed NH₃ sensor168B during normal reductant dosing operations.

As stated above, the SCR catalyst regeneration module 260 also includesan NH₃ slip feedback module 264, which monitors the ammonia measurementtaken at the SCR mid-bed NH₃ sensor 168B to determine the flow rate ofNH₃ passing through the SCR catalyst downstream of the urea deposit. TheNH₃ slip feedback module 264 can then integrate the flow rate of NH₃over the elapsed time interval from the initiation of the SCRregeneration event to determine the total amount of NH₃ passing throughthe SCR system downstream of the urea deposit.

If the total amount of ammonia reaching the SCR mid-med NH₃ sensor 168Breaches or exceeds an SCR NH₃ regeneration threshold, this indicatesthat a mass of urea deposit in the SCR piping and SCR catalyst has begunto decompose. Thus, the NH₃ slip feedback module 264 is configured toverify and confirm the presence of the urea deposit and to allow the SCRregeneration event to continue.

If, however, within a certain period of time after the initiation of theSCR regeneration event, the SCR mid-bed NH₃ sensor 168B fails to measurethe amount of additional ammonia indicative of a decomposing ureadeposit (e.g. the SCR NH₃ regeneration threshold), the SCR catalystregeneration module 260 is configured to terminate the SCR regenerationevent. This has the advantage of reducing the number of elevated heatcycles experienced by the SCR catalyst, which in turn decreases the wearand degradation of the SCR catalyst components. This also has theadvantage of reducing the amount of additional fuel required by theinternal combustion engine system to maintain its exhaustafter-treatment system in good operating condition.

The SCR NH₃ regeneration threshold can be experimentally predeterminedand customized to the specific characteristics of the internalcombustion engine and the exhaust after-treatment system (e.g. type andsize of the engine, type of SCR catalyst, etc.), with the values beingentered into a look-up table included within the NH₃ slip feedbackmodule 264. In other aspects, the SCR NH₃ regeneration threshold may bebased on information received from the urea deposit accumulationestimation module 262 that is reflective of the current estimation forthe mass of the accumulated urea deposit.

In one implementation, the total amount of ammonia reaching the SCRmid-bed NH₃ sensor 168B can include both the ammonia released from theurea deposit and the ammonia being dosed into the exhaustafter-treatment system via the reductant dosing system 151. In anotherimplementation, the amount of reductant being dosed into the exhaustafter-treatment system can be reduced or stopped during the SCRregeneration event, so as to avoid creating excess ammonia slip whichcannot be utilized by the portion of the SCR catalyst that is locatedfurther downstream from the SCR mid-bed NH₃ sensor 168B. In thesecircumstances the total amount of ammonia reaching the SCR mid-med NH₃sensor 168B may be limited primarily to the ammonia released from theurea deposit. Although the NH₃ sensor 168B has been described as amid-bed sensor, in other embodiments, the NH₃ sensor 168B can be locatedat any location downstream of the accumulated urea deposit and need notnecessarily be located in a mid-bed position of the SCR catalyst.

Exemplary Method for Reducing NO_(x) Emissions

Referring to FIG. 6, and according to one representative embodiment, amethod 400 for mitigating a urea deposit within an SCR catalyst isshown. The method 400 starts at 402 and includes determining 404 a ureadeposit accumulation rate. In some implementations, determining 404 theurea deposit accumulation rate includes operating the urea depositaccumulation estimation module 262 to calculate the instantaneous rateof urea deposition based on, at least in part, the temperature of theexhaust gas entering the SCR catalyst 152 and the dosing rate of thereductant delivery system 151 being inputted into an electronic model ofthe SCR system. In other implementations, the instantaneous rate of ureadeposition can be based on other or additional factors as is known inthe art. In some implementations the urea deposit accumulationestimation module 262 may also include current engine system operatingconditions, such as the exhaust flow rate of the exhaust gas stream, theSCR catalyst inlet NO_(x) flow rate, and SCR catalyst outlet NO_(x) flowrate, to further refine its calculation for the rate of urea deposition.The inclusion of additional measured conditions and control parametersnot specifically described herein into the calculation of the ureadeposit accumulation rate is also possible.

The method 400 also includes determining 406 the accumulated mass of theurea deposit (which can be a first estimate of the accumulated mass insome implementations) likely to be present within the SCR catalyst 252and SCR piping. In some implementations, determining 406 the accumulatedmass of the urea deposit includes operating the urea depositaccumulation estimation module 262 to integrate the rate of urea depositaccumulation over the elapsed time interval from a previous SCRregeneration event to determine the accumulated mass of the ureadeposit. Determining 406 the accumulated mass of the urea deposit canalso be accomplished using other techniques and/or processes known inthe art.

The method 400 also includes determining 408 whether or not theaccumulated mass of the urea deposit meets a urea deposit thresholdlimit. If the accumulated mass does not meet the urea deposit thresholdlimit, the urea deposit accumulation estimation module 262 cycles backto the step of determining 404 the urea deposit accumulation rate. Ifthe accumulated mass does meet the urea deposit threshold limit, themethod 400 progresses to initiating 410 an SCR regeneration event. Insome implementations, initiating 410 an SCR regeneration event includescausing the SCR catalyst regeneration module 260 to communicate an SCRcatalyst regeneration command to the thermal management module 270. TheSCR catalyst regeneration command may include the desired elevated valuefor the SCR catalyst inlet exhaust gas temperature.

After the SCR regeneration event has been initiated, the method 400includes determining 412 the amount of NH₃ passing through the SCRcatalyst 150 downstream of the urea deposit. In some implementations,determining 412 the amount of NH₃ passing through the SCR catalystincludes monitoring the level of NH₃ with the SCR mid-bed NH₃ sensor168B and the NH₃ slip feedback module 264. Using the NH₃ sensor locatedat the mid-bed location of the SCR catalyst has the advantage ofcapturing the level of NH₃ before it has had a chance to be completelyutilized in the NO_(x) reduction reaction or captured by the SCRcatalyst for storage. This allows for a more accurate indication of themass of the urea deposit, which information can be applied in futurecalculations. However, it also possible to monitor the level of NH₃which has slipped completely through the SCR catalyst using a sensorpositioned further below or downstream from the SCR catalyst, such asNH₃ sensor 168C.

In addition, determining 412 the amount of NH₃ below or downstream ofthe urea deposit can further include determining a rate of NH₃ passingthrough the SCR system downstream of the urea deposit, determining atime period which has elapsed from the initiation of the SCRregeneration event, and integrating the rate of NH₃ passing through theSCR system over the time period since the initiation of SCR regenerationevent to calculate the amount of NH₃ passing through the SCR system.

The method 400 also includes determining 414 whether or not the amountof NH₃ passing through the SCR catalyst downstream of the urea depositmeets the SCR NH₃ regeneration threshold limit. If the NH₃ leveldownstream of the urea deposit does not meet the SCR NH₃ regenerationthreshold limit, the SCR catalyst regeneration module 260 is configuredto terminate or stop 422 the SCR regeneration event and to direct thecontroller 40 to return to normal exhaust after-treatment operation.Moreover, if the measured amount of NH₃ does not meet the SCR NH₃regeneration threshold, this may indicate that the value for theaccumulated mass of the urea deposit calculated by the urea depositaccumulation estimation module 262 was inaccurate, and the furtheradjustments to the estimation process may be necessary.

If the amount of NH₃ passing through the SCR catalyst does meet the SCRNH₃ regeneration threshold limit, the method 400 includes completing theSCR regeneration event by allowing the thermal management module 270 tomaintain the desired elevated value for the SCR catalyst inlet exhaustgas temperature until it has been determined that the accumulated massof the urea deposit has been decomposed to less than a urea depositlower threshold limit. This includes determining 416 a urea depositdecomposition rate, which in some implementations includes operating theurea deposit accumulation estimation module 262 to calculate theinstantaneous rate of urea decomposition based on, at least in part, theelevated temperature of the exhaust gas entering the SCR catalyst 152.

Under these circumstances, the method 400 also includes determining 418the reduction in the accumulated mass of the urea deposit (which caninclude determining a second estimate of the accumulated mass of theurea deposit). In some implementations, determining 418 the reduction ofthe urea deposit (which can be used to determine the mass of the ureadeposit after an SCR catalyst regeneration event has started) includesoperating the urea deposit accumulation estimation module 262 tointegrate the rate of urea deposit decomposition over the elapsed timeinterval from the initiation of the SCR regeneration event to determinethe mass of the decomposing urea deposit. Accordingly, the method fordetermining the mass of the urea deposit at 406 before SCR catalystregeneration has begun (e.g., at a first time period) can be the same asor different than the method for determining the mass of the ureadeposit at 418 after the SCR catalyst regeneration has begun (e.g., at asecond time period after the first time period).

The method 400 also includes determining 420 whether or not theaccumulated mass of the urea deposit meets a urea deposit lowerthreshold limit. If the accumulated mass does not meet the urea depositlower threshold limit, the SCR regeneration event continues and the ureadeposit accumulation estimation module 262 cycles back to the step ofdetermining 416 the urea deposit decomposition rate. If the accumulatedmass does meet the urea deposit lower threshold limit, the method 400progresses to terminating 422 the SCR regeneration event.

Alternatively, the method 400 can include completing the SCRregeneration event by allowing the thermal management module 270 tomaintain the desired elevated value for the SCR catalyst inlet exhaustgas temperature for a predetermined period of time. In oneimplementation the predetermined period of time may be a fixed period oftime stored within the thermal management module 270. In anotherimplementation the predetermined period of time may be an adjustabletime period proscribed in the SCR catalyst regeneration command.Allowing the thermal management module 270 to maintain the desiredelevated temperature value at the SCR catalyst inlet for thepredetermined period of time can ensure that the urea deposit will becompletely decomposed by the end of the SCR regeneration event.

Whether the SCR regeneration event is allowed to complete normally or isterminated early, the method 400 concludes with setting 424 the timefrom the last SCR regeneration event to zero prior to ending the methodat 426. This step may be taken so that the next cycle of the method 400for mitigating a urea deposit within the SCR catalyst may start with anupdated time reference. In some implementations, setting 424 the timefrom the last SCR regeneration event to zero may be accomplished byclearing memory registers in the processor module 310 or memory module320 of the controller 40 (see, e.g., FIG. 4).

The schematic flow chart diagrams and method schematic diagramsdescribed above are generally set forth as logical flow chart diagrams.As such, the depicted order and labeled steps are indicative ofrepresentative embodiments. Other steps, orderings and methods may beconceived that are equivalent in function, logic, or effect to one ormore steps, or portions thereof, of the methods illustrated in theschematic diagrams. For example, the steps of setting 418 the time fromthe last SCR regeneration event to zero and setting 420 the value ofaccumulated mass of the urea deposit to zero may be performed at thebeginning of the process, rather than at the end.

Additionally, the format and symbols employed are provided to explainthe logical steps of the schematic diagrams and are understood not tolimit the scope of the methods illustrated by the diagrams. Althoughvarious arrow types and line types may be employed in the schematicdiagrams, they are understood not to limit the scope of thecorresponding methods. Indeed, some arrows or other connectors may beused to indicate only the logical flow of a method. For instance, anarrow may indicate a waiting or monitoring period of unspecifiedduration between enumerated steps of a depicted method. Additionally,the order in which a particular method occurs may or may not strictlyadhere to the order of the corresponding steps shown.

Many of the functional units described in this specification have beenlabeled as modules, in order to more particularly emphasize theirimplementation independence. For example, a module may be implemented asa hardware circuit comprising custom VLSI circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of executable code may, forinstance, comprise one or more physical or logical blocks of computerinstructions, which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of computer readable program code may be a singleinstruction, or many instructions, and may even be distributed overseveral different code segments, among different programs, and acrossseveral memory devices. Similarly, operational data may be identifiedand illustrated herein within modules, and may be embodied in anysuitable form and organized within any suitable type of data structure.The operational data may be collected as a single data set, or may bedistributed over different locations including over different storagedevices, and may exist, at least partially, merely as electronic signalson a system or network. Where a module or portions of a module areimplemented in software, the computer readable program code may bestored and/or propagated on in one or more computer readable medium(s).

The computer readable medium may be a tangible computer readable storagemedium storing the computer readable program code. The computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, holographic,micromechanical, or semiconductor system, apparatus, or device, or anysuitable combination of the foregoing.

More specific examples of the computer readable medium may include butare not limited to a portable computer diskette, a hard disk, a randomaccess memory (RAM), a read-only memory (ROM), an erasable programmableread-only memory (EPROM or Flash memory), a portable compact discread-only memory (CD-ROM), a digital versatile disc (DVD), an opticalstorage device, a magnetic storage device, a holographic storage medium,a micromechanical storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, and/or storecomputer readable program code for use by and/or in connection with aninstruction execution system, apparatus, or device.

The computer readable medium may also be a computer readable signalmedium. A computer readable signal medium may include a propagated datasignal with computer readable program code embodied therein, forexample, in baseband or as part of a carrier wave. Such a propagatedsignal may take any of a variety of forms, including, but not limitedto, electrical, electro-magnetic, magnetic, optical, or any suitablecombination thereof. A computer readable signal medium may be anycomputer readable medium that is not a computer readable storage mediumand that can communicate, propagate, or transport computer readableprogram code for use by or in connection with an instruction executionsystem, apparatus, or device. Computer readable program code embodied ona computer readable signal medium may be transmitted using anyappropriate medium, including but not limited to wireless, wireline,optical fiber cable, Radio Frequency (RF), or the like, or any suitablecombination of the foregoing

In one embodiment, the computer readable medium may comprise acombination of one or more computer readable storage mediums and one ormore computer readable signal mediums. For example, computer readableprogram code may be both propagated as an electro-magnetic signalthrough a fiber optic cable for execution by a processor and stored onRAM storage device for execution by the processor.

Computer readable program code for carrying out operations for aspectsof the present invention may be written in any combination of one ormore programming languages, including an object oriented programminglanguage such as Java, Smalltalk, C++ or the like and conventionalprocedural programming languages, such as the “C” programming languageor similar programming languages. The computer readable program code mayexecute entirely on the user's computer, partly on the user's computer,as a stand-alone software package, partly on the user's computer andpartly on a remote computer or entirely on the remote computer orserver. In the latter scenario, the remote computer may be connected tothe user's computer through any type of network, including a local areanetwork (LAN) or a wide area network (WAN), or the connection may bemade to an external computer (for example, through the Internet using anInternet Service Provider).

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention. Thus,appearances of the phrases “in one embodiment,” “in an embodiment,” andsimilar language throughout this specification may, but do notnecessarily, all refer to the same embodiment. Similarly, the use of theterm “implementation” means an implementation having a particularfeature, structure, or characteristic described in connection with oneor more embodiments of the present disclosure, however, absent anexpress correlation to indicate otherwise, an implementation may beassociated with one or more embodiments.

The present disclosure may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the disclosure is, therefore,indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. A method for mitigating a urea deposit within aselective catalytic reduction (SCR) system in exhaust receivingcommunication with an exhaust gas stream produced by an engine system,the exhaust gas stream flowing from an engine of the engine system to atailpipe of the engine system, the method comprising: determining a massof an accumulated urea deposit present within an SCR system, the SCRsystem including an SCR catalyst and SCR piping; comparing the mass ofthe accumulated urea deposit with a deposit upper threshold limit;initiating an SCR regeneration event when the mass of the accumulatedurea deposit is greater than the deposit upper threshold limit;determining an amount of NH₃ passing through the SCR system downstreamof the urea deposit; comparing the amount of NH₃ passing through the SCRsystem with an NH₃ regeneration threshold limit; and terminating the SCRregeneration event when the amount of NH₃ passing through the SCR systemis less than the SCR NH₃ regeneration threshold.
 2. The method of claim1, wherein determining the mass of an accumulated urea depositcomprises: determining a rate of urea deposit accumulation within theSCR system; determining a first time period, the first time period beingthe time elapsed from a previous SCR regeneration event; and integratingthe rate of urea deposit accumulation within the SCR system over thefirst time period to calculate the mass of the accumulated urea deposit.3. The method of claim 1, wherein determining the amount of NH₃ passingthrough the SCR system downstream of the urea deposit comprises:determining a flow rate of NH₃ passing through the SCR system downstreamof the urea deposit; determining a second time period, the second timeperiod being the time elapsed from the initiation of the SCRregeneration event; and integrating the rate of NH₃ passing through theSCR system over the second time period to calculate the amount of NH₃passing through the SCR system.
 4. The method of claim 3, furthercomprising measuring the flow rate of NH₃ passing through the SCR systemwith an NH₃ sensor positioned downstream of the urea deposit.
 5. Themethod of claim 4, wherein the NH₃ sensor is an SCR catalyst NH₃ mid-bedsensor.
 6. A method for mitigating a urea deposit within a selectivecatalytic reduction (SCR) system in exhaust receiving communication withan exhaust gas stream produced by an engine system, the exhaust gasstream flowing from an engine of the engine system to a tailpipe of theengine system, the method comprising: determining a mass of anaccumulated urea deposit present within the SCR system, the SCR systemincluding an SCR catalyst and SCR piping upstream of the SCR catalyst;comparing the mass of the accumulated urea deposit with a deposit upperthreshold limit; initiating an SCR regeneration event when the mass ofthe accumulated urea deposit is greater than the deposit upper thresholdlimit; determining an amount of NH₃ passing through the SCR catalystdownstream of the urea deposit; comparing the amount of NH₃ passingthrough the SCR catalyst with an NH₃ regeneration threshold limit;maintaining the SCR regeneration event when the level of NH₃ passingthrough the SCR catalyst is greater than the NH₃ regeneration thresholdlimit; comparing the mass of the accumulated urea deposit with a depositlower threshold limit; and terminating the SCR regeneration event whenthe mass of the accumulated urea deposit is less than the deposit lowerthreshold limit.
 7. The method of claim 6, wherein determining the massof the accumulated urea deposit comprises: determining a rate of ureadeposit accumulation within the SCR system; determining a first timeperiod, the first time period being the time elapsed from a previous SCRregeneration event; and integrating the rate of urea depositaccumulation within the SCR system over the first time period tocalculate the mass of the accumulated urea deposit.
 8. The method ofclaim 6, wherein determining the amount of NH₃ passing through the SCRcatalyst downstream of the urea deposit comprises: determining a flowrate of NH₃ passing through the SCR catalyst downstream of the ureadeposit; determining a second time period, the second time period beingthe time elapsed from the initiation of the SCR regeneration event; andintegrating the rate of NH₃ passing through the SCR catalyst over thesecond time period to calculate the amount of NH₃ passing through theSCR catalyst.
 9. The method of claim 8, further comprising measuring theflow rate of NH₃ passing through the SCR catalyst downstream of the ureadeposit with an NH₃ sensor positioned downstream of the urea deposit.10. The method of claim 9, wherein determining the mass of theaccumulated urea deposit comprises: determining a rate of urea depositdecomposition within the SCR system; and integrating the rate of ureadeposit decomposition within the SCR system over the second time periodto calculate the mass of the accumulated urea deposit.
 11. A system formitigating a urea deposit within a selective catalytic reduction (SCR)catalyst and SCR piping in exhaust receiving communication with anexhaust gas stream produced by an engine system, the exhaust gas streamflowing from an engine of the engine system to a tailpipe of the enginesystem, the system comprising: a reductant delivery system configured toinject reductant into the exhaust gas stream upstream of the SCRcatalyst; an SCR NH₃ sensor in exhaust gas receiving communication withthe exhaust gas stream; and a controller having an SCR catalystregeneration module in electrical communication with the reductantdelivery system and the SCR NH₃ sensor, the SCR catalyst regenerationmodule comprising: a urea deposit accumulation module operable todetermine a mass of an accumulated urea deposit present within the SCRcatalyst and SCR piping from at least one input received from thereductant delivery system; and an NH₃ slip feedback module operable todetermine an amount of NH₃ passing through the SCR catalyst downstreamof the urea deposit from sensor inputs received from the SCR NH₃ sensor;wherein the SCR catalyst regeneration module is operable to initiate anSCR regeneration event when the determined mass of the accumulated ureadeposit is greater than a deposit upper threshold limit, and wherein theSCR catalyst regeneration module is operable to terminate the SCRregeneration event when the level of NH₃ passing through the SCRcatalyst after initiation of the SCR regeneration event is less than anSCR NH₃ regeneration threshold.
 12. The system of claim 11, wherein theSCR NH₃ sensor is located downstream of the accumulated urea deposit.13. The system of claim 11, further comprising: an exhaust flow ratesensor in exhaust gas receiving communication with the exhaust gasstream; an SCR catalyst inlet NO_(x) flow rate sensor in exhaust gasreceiving communication with the exhaust gas stream and locatedproximate an inlet of the SCR catalyst; and an SCR catalyst outletNO_(x) flow rate sensor in exhaust gas receiving communication with theexhaust gas stream and located proximate an outlet of the SCR catalyst,wherein the exhaust flow rate sensor, the SCR catalyst inlet NO_(x) flowrate sensor, and the SCR catalyst outlet NO_(x) flow rate sensor areeach in electrical communication with the SCR catalyst regenerationmodule and operable to provide sensor inputs to the urea depositaccumulation module for determining the mass of the accumulated ureadeposit.
 14. The system of claim 11, wherein the inputs received fromthe reductant delivery system comprises a reductant dosing flow signal.